ABSTRACT Although a strong inverse association has been observed between tubal ligation and epithelial ovarian cancer risk, it is unclear how long the protection from a tubal ligation lasts, or whether the inverse association is stronger for, or observed only in, specific subgroups of women. Further, the underlying biological mechanisms responsible for the protective effects of this procedure are unclear. Therefore, in this application, we will conduct a detailed evaluation of tubal ligation with ovarian cancer risk. In this study, we propose to re-examine tubal ligation as an important risk factor for epithelial ovarian cancer, and specifically, to evaluate a modifying role of both surgery-related and other lifestyle choices (e.g., oral contraceptive use, genital talc use, and individual risk of ovarian cancer) as well as tumor characteristics. In addition, we plan to assess whether the cancer protective effects of tubal ligation are mediated via inflammatory processes or MUC1-associated immunity. Using prospectively collected questionnaire data and paraffin-embedded ovarian tumor tissue already collected from women in the Nurses' Health Study (NHS) and NHSII, two large prospective cohort studies initiated in 1976 (n=121,700) and 1989 (n=116,430), respectively, we propose to investigate a role of tubal ligation and potentially important mechanisms in detail. Over the course of follow-up (1976-2010 in NHS and 1989-2009 in NHS II) we expect to accrue 1,215 confirmed cases of epithelial ovarian cancer, of which 506 will have tumor tissue available. With the unique and prospectively-collected resources of these two cohorts, we will identify populations who can most benefit from tubal ligation in relation to ovarian cancer risk as well as elucidate the timing of when the surgery may be most protective - this could lead to important changes in prevention recommendations that could be instituted in the short term. In addition we will examine whether the inverse association of tubal ligation varies by histologic subtype, putative cell of origin (e.g., for tumors derived from the distal fallopian tube rather than the ovarian surface epithelium), or expression of inflammatory markers in the tumor (phosphorylated STAT3 (pSTAT3), pSTAT5, COX-2, MUC1). Given the weight of evidence that tubal ligation reduces risk of this disease, it is imperative to understand why and how this procedure confers protection, as well as whether characteristics of the surgery, woman, or tumor impact the effectiveness of tubal ligation. If these questions are addressed, there are potential implications for improving ovarian cancer prevention recommendations regarding tubal ligation. Further, examining the biological mechanisms underlying the protective effect of this common procedure will hopefully lead to other avenues of prevention that may be less invasive than having a tubal ligation. PUBLIC HEALTH RELEVANCE: PROJECT NARRATIVE Tubal ligation reduces risk of epithelial ovarian cancer, in this application we will examine why and how this procedure confers protection, as well as whether characteristics of the surgery, woman, or tumor impact the effectiveness of tubal ligation. This can be used to improve prevention recommendations for women at high risk of this disease.